The human genome project has succeeded in sequencing most regions of human DNA. Work to identify the genes and sequence alterations associated with disease continues at a rapid pace. Linkage studies are used to associate phenotype with genetic markers such as simple sequence repeats or single nucleotide polymorphisms (SNPs) to identify candidate genes. Sequence alterations including SNPs, insertions, and deletions that cause missense, frameshift, or splicing mutations then may be used to pinpoint the gene and the spectrum of responsible mutations.
However, even when the genetic details become known, it is difficult to use this knowledge in routine medical practice, in large part because the methods to analyze DNA are expensive and complex. When costs are significantly lowered and the methods dramatically simplified, it is expected that DNA analysis will become accessible for use in everyday clinical practice for effective disease detection and better treatment. Ideal DNA analysis is rapid, simple, and inexpensive.
When a disease is caused by a limited number of mutations, or when a few sequence alterations constitute a large proportion of the disease cases, direct genotyping is feasible. Traditional methods range from classical restriction digestion of PCR products to closed-tube fluorescent methods. Closed-tube methods of DNA analysis can be simple to perform. Once PCR is initiated, no further reagent additions or separations are necessary. However, when one allele is present in small quantities, that allele may be difficult to detect.
Sequencing is currently the gold standard for identifying sequence variation. Even though costs are decreasing, sequencing is still a complex process that is not rapid, simple, or inexpensive when applied to specific genetic diagnosis or pharmacogenetics. Standard sequencing requires seven steps: 1) amplification by PCR, 2) clean up of the PCR product, 3) addition of cycle sequencing reagents, 4) cycle sequencing for dideoxy termination, 5) clean up of the termination products, 6) separation by electrophoresis, and 7) data analysis. This complexity can be automated and has been in some sequencing centers, but sequencing still remains much more complex than the methods of the present invention. Further, when large or multiple genes are analyzed, often over 90% of the sequenced products come back normal. Moreover, current sequencing methods fail to identify low copy alleles, particularly when the alleles are present in an allele fraction of less than 20%. Identifying the presence of these low-copy alleles is important in a number of settings, illustratively in identifying the presence of certain oncogene mutations or changes in tumor samples or peripheral fluids such as blood. The presence or absence of such alleles can be particularly important for the selection of treatment protocols, illustratively with detection/confirmation of common somatic mutations (p53, EGFR, BRAF) and early identification of mutant bacterial infections (e.g., malaria) where standard therapies are contraindicated. Other examples of low levels of mutant alleles that can be found against a predominantly wild-type background are in mitochondrial DNA and fetal DNA present within maternal circulation. In addition, detection of low levels of epigenetic mutations is desired. For example, it was recently found that BRCA1 promoter methylation between 1 and 10% was associated with breast cancer phenotypes (Snell et. al., 2008, Breast Cancer Research)
PCR-based techniques for enriching the proportion of minority alleles and mutations in a sample are known. When the genotype of the mutation is unknown, COLD-PCR can be used (Li J, et al., Nat Med 2008; 14:579-84). This technique can detect down to a 1:100 ratio of mutant allele to wild type. However, because it is nonspecific and detects any variant that occurs, additional analysis is necessary to identify the products. For enriching known SNPs, some of the most popular techniques are ARMS (Newton C R, et al., Nucleic Acids Res 1989; 17:2503-16), PNA-mediated PCR (Nielsen P E, et al., Science 1991; 254:1497-500; Dabritz J, et al., Br J Cancer 2005; 92:405-12), LNA-mediated WTB-PCR (Dominguez P L, Kolodney M S. Wild-type blocking polymerase chain reaction for detection of single nucleotide minority mutations from clinical specimens. Oncogene 2005; 24:6830-4), MAMA-PCR (Cha R S, et al., PCR Methods Appl 1992; 2:14-20), TaqMAMA (Li B, et al., Genomics 2004; 83:311-20; Easterday W R, et al., Biotechniques 2005; 38:731-5), and SCORPION® primers (Whitcombe D, et al., Nat Biotechnol 1999; 17:804-7). These methods detect mutations by allele specific PCR, noting differences in quantification cycle (ACq) and can detect a 1:1000 ratio of mutant allele to wild type.
High resolution melting was introduced as a homogeneous method of scanning PCR amplicons for heterozygous sequence variants. See, e.g., U.S. Pat. Nos. 7,387,887 and 7,582,429, herein incorporated by reference in their entirety. Based on the use of dsDNA saturating dyes, high resolution melting is capable of detecting SNPs and insertions/deletions in amplicons up to 400 bp at a sensitivity >99%. Since its introduction in 2003, additional applications for high resolution melting have been developed, including genotyping for known sequence variants using small amplicons or unlabeled probes (LUNAPROBES™) Unlabeled probes are blocked on the 3′-end to prevent extension during PCR and may use a dsDNA saturation dye, illustratively LCGREEN® Plus (Idaho Technology, Salt Lake City, Utah), to discriminate the genotype of the allele based on probe melting temperature (Tm). The probe sequence can be designed to match either allele and is based on maximizing the ΔTm between the perfect match and mismatched probe. For more information on the use of unlabeled probes, see U.S. Pat. No. 7,387,887, already incorporated by reference.
It has been found that the probes themselves may be used to bias amplification of low fraction alleles. Examples 1-5 below are presented using unlabeled probes. Examples 6-8 are presented using Snapback primers. With a Snapback primer, the primer comprises a probe element specific for a locus of the target nucleic acid and a template-specific primer region, wherein the probe element is 5′ of the template-specific primer region. After amplification, the probe element may hybridize to the locus to form a hairpin in an intramolecular reaction or may hybridize to its complement strand in an intermolecular reaction. Thus, a Snapback primer incorporates the probe element into the same oligonucleotide as the primer. Snapback primers may be labeled, but they are often used unlabeled, in a manner similar to unlabeled probes. See WO 2008/109823 (PCT/US08/56217), incorporated herein in its entirety for a detailed discussion of Snapback primers.
While unlabeled probes and unlabeled Snapback primers are used herein, it is understood that the probes may be labeled as well. When unlabeled probes are used they tend to be somewhat larger than other probes (often 25-30 bp) to generate sufficient fluorescent signal from the dsDNA binding dye, and due to this length they are well suited to bias preferentially the amplification of the mismatched allele. The probe (whether unlabeled probe, Snapback probe element, or other probe) is matched to the higher fraction allele, and “allele amplification bias” is empirically determined by setting the annealing temperature (or extension temperature, if used) of PCR somewhere between the Tm of the perfectly matched and somewhat below the Tm of the mismatched probe, illustratively at the Tm of the lower allele or about half way between the Tms, depending on how much melting peaks for the two alleles overlap. At this mid-Tm annealing temperature, the perfectly matched probe is bound to its target (often the wild type allele) and is stable enough to retard amplification. In one embodiment, rapid cycle PCR performed on the LIGHTSCANNER® 32 (“LS32”, Idaho Technology, Inc.) was used to aid the stringency of the target annealing temperature and hinder amplification of the wild type allele, although it is understood that other instruments may be suitable. An exo− polymerase may also be used to avoid probe digestion and aid in biasing amplification of the lower Tm allele.